One iteration of polling is always performed even when polling is
disabled. This is done because:
1. Userspace polling is cheaper than making a syscall. We might get
lucky.
2. We must poll once more after polling has stopped in case an event
occurred while stopping polling.
However, there are downsides:
1. Polling becomes a bottleneck when the number of event sources is very
high. It's more efficient to monitor fds in that case.
2. A high-frequency polling event source can starve non-polling event
sources because ppoll(2)/epoll(7) is never invoked.
This patch removes the forced polling iteration so that poll_ns=0 really
means no polling.
IOPS increases from 10k to 60k when the guest has 100
virtio-blk-pci,num-queues=32 devices and 1 virtio-blk-pci,num-queues=1
device because the large number of event sources being polled slows down
the event loop.
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://lore.kernel.org/r/20200305170806.1313245-2-stefanha@redhat.com
Message-Id: <20200305170806.1313245-2-stefanha@redhat.com>
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